there are actually two 'double miles' promos out now.(that i know of) There s the double miles promo from using your US Mastercard, and the invite-only Dividend miles double miles offer.
The US Mastercard gives you double miles to (most) european cities and either 10 or 15% off the tix. The double miles do not count toward your preferred status...there are also blackout dates around the holidays (both dep and return dates must be eligable for the promo to recieve credit).
The invite only offer only includes one international city route...PHL-LGW. Those double miles DO count towards preferred status. Blackout dates around the holidays, and there are a couple other strings (dep only on mon-thr, adv purchase, etc....).